Since the inception of the computer, steady increases in computing performance have been enabled by making transistors (devices) smaller. In the case of integrated circuits, reduced device size increases the number of transistors and decreases the distance between them collectively resulting in increases in processing power and speed. Continued advances in computer power depend on making devices that approach the nanoscale. Such devices present numerous difficulties with respect to manufacturing, lithography and indeed physical laws that make the use of silicon transistors at the nanoscale subject to quantum tunneling effects. Because of these problems current CMOS transistors cannot easily be scaled to these dimensions.
Contemporary integrated circuits utilize a type of random access memory (RAM) known as DRAM (Dynamic Random Access Memory) for storing a bit of information as an electric charge. A typical DRAM comprises a metal oxide semiconductor (MOS) transistor for which the drain of said MOS transistor is connected to a capacitor, most often the gate of an adjacent transistor. The capacitor stores the charge and in this way, a bit of information can be retained and later recalled. The amount of charge that can be stored in a capacitor is a property dependent upon the plate area of the capacitor. DRAM requires frequent refresh to replenish charge that leaches from the capacitor. It remains a significant challenge to squeeze high performance from transistors of remarkably small dimensions; in particular, retaining charge in capacitors of diminishing area and reducing refresh frequencies represent nontrivial problems in the art.
Molecular electronics has the potential to augment or completely replace conventional devices with electronic elements fashioned from nanoscale entities. Such elements can be altered by externally applied voltages and have the potential to scale from micron-sized dimensions to nanometer-scale dimension with little change in the device concept. The molecular switching elements can be formed by inexpensive solution techniques. The self-assembled switching elements may be integrated on top of a silicon integrated circuit so that they can be driven by conventional silicon electronics in the underlying substrate. To address the switching elements, interconnections or wires are used.
Prior proposed nanoscale devices have invoked a number of methods, features and approaches. It has been suggested that fine scale lithography using X-rays, electrons, ions, scanning probes, or stamping could be used to print the requisite pattern for device components on a chip. Alternatively, these techniques could be used to directly carve such features into a chip. Another method utilizes chemical synthesis to link molecular device components via covalent bonds. The major problem with fine scale lithography is that the wafer on which the devices are built must be aligned to within a small fraction of the size of the device features in at least two dimensions for several successive stages of lithography, followed by etching or deposition to build the devices. This level of control does not scale well as device sizes are reduced to nanometer scale dimensions. The main problem with direct writing is that it is a serial process, and direct writing a wafer full of complex devices, each containing trillions of components, could well require years. Finally, with respect to chemical synthesis, high information content molecules are typically macromolecular structures such as proteins or DNA, and both have extremely complex, and, to date, unpredictable secondary and tertiary structures that cause them to twist into helices, fold into sheets, and form other complex 3D structures that will have a significant and usually deleterious effect on their desired electrical properties as well as make interfacing them to the outside world impossible.
Molecular electronic devices comprising crossed wire switches hold promise for future electronic and computational devices. Thin single or multiple atomic layers can be formed, for example, by Langmuir-Blodgett techniques or self-assembled monolayers at a specific site. A crossed wire switch may comprise two wires, or electrodes, for example, with a molecular switching species between the two electrodes.
The so-called crossbar technology of Hewlett-Packard has explored many of the issues associated with nanoscale electronic devices.
U.S. Pat. No. 6,459,095 B1 to Heath et al discloses a route to the fabrication of electronic devices wherein the devices consist of two crossed wires sandwiching an electrically addressable molecular species. The device can be used to produce crossbar switch arrays, logic devices, memory devices, and communication and signal routing devices. The route of the '095 patent enables construction of molecular electronic devices on a length scale that can range from micrometers to nanometers via a chemical assembly procedure.
U.S. Pat. No. 6,432,740 B1 to Chen discloses a method of fabricating a molecular electronic device or crossbar memory that involves forming a top electrode after forming the molecular switch. The device comprises at least one pair of crossed wires and a molecular switch film therebetween. The method comprises: (a) forming at least one bottom electrode on a substrate by first forming a first layer on the substrate and patterning the first layer to form the bottom electrode by an imprinting technique; (b) forming the molecular switch film on top of the bottom electrode; (c) optionally forming a protective layer on top of the molecular switch film to avoid damage thereto during further processing; (d) coating a polymer layer on top of the protective layer and patterned the polymer layer by the imprinting method to form openings that expose portions of the protective layer; and (e) forming at least one top electrode on the protective layer through the openings in the polymer layer by first forming a second layer on the polymer layer and patterning the second layer. The imprinting method can be used to fabricate nanoscale patterns over a large area at high speeds acceptable in industrial standards.
U.S. patent application U.S. 2002/0176276 A1 to Zhang et al discloses molecular systems for electric field activated switches, such as a crossed-wire device or a pair of electrodes to which the molecular system is linked by linking moieties. The crossed-wire device comprises a pair of crossed wires that form a junction where one wire crosses another at an angle other than zero degrees and at least one connector species connecting the pair of crossed wires in the junction. The connector species comprises the molecular system, which has an electric field induced band gap change, and thus a change in its electrical conductivity that occurs via one of the following mechanisms: (1) molecular conformation change; (2) change of extended conjugation via chemical bonding change to change the band gap; or (3) molecular folding or stretching.
U.S. Pat. No. 6,512,119 B2 to Bratkovski et al discloses crossed-wire devices that comprise a pair of crossed wires that form a junction where one wire crosses another at an angle other than zero degrees and at least one connector species connecting the pair of crossed wires lies within the junction. The junction has a function dimension in nanometers, wherein at least one connector species and the pair of crossed wires form an electrochemical cell.
U.S. Pat. No. 5,772,905 to Chou discloses a lithographic method and apparatus for creating ultra-fine (sub-25 nm) patterns in a thin film coated on a substrate in which a mold having at least one protruding feature is pressed into a thin film carried on a substrate. The protruding feature in the mold creates a recess of the thin film. The mold is removed from the film. The thin film then is processed such that the thin film in the recess is removed exposing the underlying substrate. Thus, the pattern in the mold is replaced in the thin film, completing the lithography. The patterns in the thin film will be, in subsequent processes, reproduced in the substrate or in another material which is added onto the substrate.
U.S. Pat. No. 6,314,019 B1 to Kuekes et al discloses a molecular-wire crossbar interconnect for signal routing and communications between a first level and a second level in a molecular-wire crossbar. The molecular wire crossbar comprises a two-dimensional array of a plurality of nanometer-scale switches. Each switch is reconfigurable and self-assembling and comprises a pair of crossed wires which form a junction where one wire crosses another and at least one connector species connecting the pair of crossed wires in the junction. The connector species comprises a bi-stable molecule. Each level comprises at least one group of switches and each group of switches comprises at least one switch, with each group in the first level connected to all other groups in the second level in an all-to-all configuration to provide a scalable, defect-tolerant, fat-tree networking scheme.
One prior art method, as described in Reed et al. Applied Physics Letters, 2001, vol. 78, no. 23, pg. 3735-3737, involves a nanoscale electronic device made by depositing a second electrode by sputtering or chemical vapor deposition (CVD) atop a molecular electronic component that lies upon a first electrode. Both sputtering and CVD techniques involve the use of energetic metal atoms that may easily penetrate and destroy the delicate mono- or multi-layer of molecular electronic component and/or establish a contact through the molecular electronic component. The resulting direct interelectrode connection and consequent electrical short gives rise to a shorted, i.e. defective, electronic device.
Despite advances to date in making nanoscale electronic devices, there remains a need for methods of making nanoscale electronic devices wherein the method does not introduce features that can give rise to short circuits in the devices made. In addition, it remains desirable to provide methods of making nanoscale devices comprising molecules that preserve both the integrity of the molecular component and the surfaces to which they are adjoined. Furthermore, there remains a need for methods of making nanoscale electronic devices that are uniform with respect to device dimension and smoothness at the nanoscale regime. Notwithstanding the previous work, the need for methods of effectively and efficiently making nanoscale devices remains less than fully solved.